It is known that all metallic structures that come in contact with a medium having the properties of an electrolyte are susceptible to the phenomenon of corrosion. Such corrosion tends to destroy the metallic structure and, depending upon the particular corrosive conditions existing, destruction of the metallic structure may occur within a longer or shorter period of time. In many instances significant damage to the metallic structure may occur within a short period of time even though destruction of the metallic structure has not yet occurred. There are many structures subject to corrosion damage, including bridges, pipes, storage tanks, reinforcing steel of concrete structures, structural steel and piles. In most cases the electrolytes for such structures comprise water with dissolved salts and moist soils.
In order to prevent/minimize corrosion, cathodic protection systems (CPSs) are often employed. CPS design is influenced by numerous factors, including the type of metal to be protected, properties of the electrolyte (chemical, physical and electrical), temperatures, presence or absence of bacteria, shape of the structure, design life, constructability and maintainability. Cathodic protection (CP) is often applied to coated structures, with the coating providing a primary form of protection and an electric current providing a secondary protection.
In general, CPSs operate by utilizing an electrical current to oppose a corrosion current between the structure being protected and an electrolyte. There are basically two known systems for generating opposing electrical currents, “sacrificial systems” and “impressed current systems.”In sacrificial systems, the current is supplied by another metal which is galvanically more reactive than the metal of the structure. For example, metals such as aluminum, magnesium and zinc are galvanically more active than steel and are used as “sacrificial anodes” to protect steel structures. In impressed current systems, a consumable metal is used to drain direct current (DC) supplied from an external source into the electrolyte, which passes to the structure to be protected. The parts from which the current is drained are called “anodes” and the protected structure is called a “cathode”. In both sacrificial and impressed current systems of cathodic protection, a metallic path between the anode and the cathode is essential for flow of current to protect the structure.
As stated above, in impressed current cathodic protection, a DC current is applied to a buried structure and flows onto the structure at coating defects. The applied current changes the voltage across the metal/soil interface. This change in voltage changes the electrochemical state of the structure to the extent that corrosion ceases.
The voltage across the metal/soil interface can be measured by monitoring the voltage difference between the structure and a second dissimilar metal (reference electrode) in contact with the soil. By monitoring the voltage difference it can therefore be determined if corrosion protection of the structure is being achieved. The cathodic protection circuits may be monitored at “test stations,” e.g., wire connections to the buried structure that terminate in some way above ground. If the structure is a pipeline, test stations are installed at regular intervals on a pipeline (typically one-mile apart) and often at road crossings for accessibility. A portable pipe-to-soil measurement unit is used to measure the voltage difference between the pipeline and the reference electrode at each test station by having an individual visit each test station along the pipeline and take manual measurements at each test station. The measured voltage level is termed a “pipe-to-soil” potential.
The pipe-to-soil potential measurement unit includes a volt meter having a test lead extending from the volt meter to the wire connection extending from the pipeline. The pipe-to-soil potential measurement unit also includes a reference electrode in contact with the ground above the buried pipeline. The reference electrode has an electrode potential that does not vary such that it supplies the pipe-to-soil potential measurement unit with a stable reference potential. The reference electrode typically includes a copper rod in a copper sulfate solution. The volt meter then measures the potential difference between these two half-cells and the value of this potential difference is the pipe-to-soil potential. The pipe-to-soil potential will vary depending upon the current that is being supplied to the pipeline by virtue of one or more of the cathodic protection systems along a particular length of the pipeline.
A cross-country pipeline will have numerous cathodic protection circuits with power source installations, or rectifiers, on each circuit to distribute the impressed current along the entire length of the pipeline. The spacing between cathodic protection circuits depends on many factors including soil conditions and coating quality, but typical spacing is approximately 10 to 30 miles apart.
Typically if there is no current being supplied to the pipeline by cathodic protection circuits, the pipe-to-soil potential is approximately −0.5 to −0.6 volts. This is referred to as the “static” or “native” potential. The “static” or “native” potential may be measured after the cathodic protection circuits have been off for such a period of time that the current from the cathodic protection circuits no longer influences the pipe-to-soil potential. As current is supplied to the pipeline by the cathodic protection circuits, the pipe-to-soil potential will tend toward the negative, preventing corrosion from forming. This is referred to as the “on” potential. Various criteria are used in the industry to determine if the pipe-to-soil potential has been shifted sufficiently negative to prevent corrosion. The most common criterion is that the potential difference, while the cathodic protection circuits are switched on, is more negative than −0.85V.
Each cathodic protection circuit with a rectifier will have an influence along a particular length of the pipeline, i.e., an area of influence. The current difference between a particular cathodic protection circuit being on or off determines the influence of that cathodic protection circuit on the pipe-to-soil potential at any particular point along the pipeline. When a cathodic protection circuit is turned off, there is a drop in current flow to the pipeline causing an increase in the pipe-to-soil potential measured by the pipe-to-soil measurement unit. This change in pipe-to-soil potential or influence of a particular rectifier can be measured with a portable pipe-to-soil measurement unit at each test station by measuring the pipe-to-soil potential with a rectifier switched on and measuring it with the rectifier switched off. The difference between these two values is the influence that the switched on rectifier has on the pipe-to-soil potential. The influence will depend upon the size of the rectifier and how much power it is sending into the soil as well as the local soil condition for current flow. The condition of the coating on the pipeline is also a factor.
By measuring the influence of each rectifier at each test station, it is therefore possible to obtain a profile of the influence of each rectifier along the pipeline. The information obtained from measuring the influence of individual rectifiers is used for specialized troubleshooting of cathodic protection systems and it is not typically used as a routine monitoring procedure.
Routine monitoring of cathodic protection systems is important to ensure that the protected structure remains in good condition. Basic routine monitoring of CPSs determines the measured status of the CPSs and includes 1) checking that all rectifiers are functioning and supplying current to the structure and 2) checking that the pipe-to-soil potential with all the rectifiers in the “on” position, is maintained at a value more negative than −0.85V using a copper/copper sulfate reference electrode at all test stations along the length of the structure. If, when using a copper/copper sulfate reference electrode, the pipe-to-soil potential is more negative than −0.85V, the steel pipeline is receiving corrosion protection.
Instead of physically visiting rectifiers to check that they are functioning and supplying current to the structure, devices known as “remote monitoring units” or RMUs may be used to remotely monitor the rectifiers from a central location. These devices use some form of communication method to automatically transmit the measured status of a rectifier to a central location. A typical remote monitoring device for rectifiers using Low Earth Orbital (LEO) satellites as the communication link is described in U.S. Pat. No. 5,785,842.
Typically, RMUs are installed inside each rectifier of a cathodic protection circuit. This allows the RMU to remotely read the status of the rectifier and the pipe-to-soil potential at the rectifier. The most common problem associated with the remote monitoring devices is failures that occur as a result of electrical surges, either from the alternating current (AC) supply within the rectifier or through the connection to the pipeline or the connection to the anodes. The remote monitoring described above also has the disadvantage that information on the cathodic protection (CP) status at the rectifier is limited; because the rectifier is the point source of current being supplied to the pipeline, and therefore the pipe-to-soil potential at that point will invariably be satisfactory. Pipe-to-soil potentials of −2V to −3V are very typical. As a result, the CP engineer has to rely on manual pipe-to-soil readings at test stations to ensure that a good CP profile exists along the pipeline.
Because cathodic protection remote monitoring devices installed in rectifiers do not monitor the pipe-to-soil potential along the pipeline, manual testing to determine the pipe-to-soil potentials along the pipeline is necessary in addition to monitoring the rectifier itself to ensure proper functioning of CPSs. Typically, manual pipe-to-soil potential data at test stations is limited to monthly or annual evaluations, so CPSs may be incorrectly preventing corrosion for some period of time before damage is detected. Furthermore, remote monitoring devices are susceptible to failure caused by electrical surges, thereby decreasing the usefulness of these devices to monitor the proper functioning of the rectifiers. Manual testing of pipe-to-soil potentials along the pipeline and repairing remote monitoring units damaged by electrical surges is expensive, time-consuming and produces dated information. Despite the known deficiencies possessed by current RMUs, to date no one has developed an arrangement that correctly obtains information about the pipe-to-soil potential along the pipeline, while simultaneously determining the status of rectifiers and also preventing failure from electrical surges. More specifically, to date no one has developed a remote monitoring arrangement that utilizes rectifier influence data to determine the status of rectifiers.
One way of protecting components susceptible to damage by electrical surges is to electrically disconnect the components from the source of the surge during times when the device is not used. A normal switch (e.g. a relay) may not be sufficient because if the surge is big enough, it will jump across the air gap or arcing will occur between one contact and the relay circuitry. A disconnect device has been described in U.S. Pat. No. 5,453,899 that senses the presence of an electrical storm and then unplugs the electrical apparatus from the AC power if an electrical storm is detected. In this case, arcing is avoided by placing a dielectric material between the contact points after the apparatus is disconnected.
In order to determine the influence from individual rectifiers, it is necessary to (1) switch each of the rectifiers off and measure the pipe-to-soil potential at each test station and then (2) switch each of the rectifiers back on and measure the pipe-to-soil potential at each test station. The shift of the pipe-to-soil potential from off to on for each individual rectifier can then be determined at each test station. Instead of manually switching each rectifier off and on, it is common in the CP industry to install a current interrupter into the rectifier under investigation. By installing an interrupter into a rectifier, it is therefore possible to visit each of the test stations and measure the influence of the rectifier being interrupted. A current interrupter is a device that interrupts the output from the rectifier in a periodic fashion and it is typically programmable so that the length of the on and off cycles can be adjusted. The influence of other rectifiers is then measured by moving the interrupter to each of these rectifiers in turn and re-visiting each of the test stations. This cycle is repeated for each cathodic protection circuit influencing that length of the pipeline. Thus if there are four cathodic protection circuits affecting a particular length of the pipeline, this cycle will need to be performed four times until the influence of each one of the rectifiers at the cathodic protection circuits has been measured at each of the test stations along the length of the pipeline.
Current interrupters are also used to determine “instant off” pipe-to-soil potentials at test stations. If a pipe-to-soil potential is measured with rectifiers switched on, there is an inherent error in the measured value because of a voltage drop that occurs due to current flow through the soil. To minimize the effect of rectifier current, the rectifiers are turned off and the pipe-to-soil potential is immediately measured using the voltmeter (typically within 1 second). This value is referred to as the “interrupted off” or “instant off” potential. By measuring at “instant off,” any error introduced due to the current of the rectifiers is minimized. This is achieved by installing current interrupters into each influencing rectifier and programming these interrupters to switch off and on at the same time. The interrupters generate “on” and “off” cycles for all of the influencing rectifiers. Some of the available interrupters only have fixed “on” and “off” cycles, while others are programmable and the length of the “off” and of the “on” cycle can be adjusted. Some models also have the ability to program a start and stop time for the interruption cycle. In all the equipment currently available, all of the interrupters switch on and off at the same time. Synchronization of the various interrupters is achieved through synchronizing their internal clocks, often using satellite time signals. U.S. Pat. No. 4,356,444 describes a plurality of interrupters which switch rectifiers on and off in unison. Each interrupter is synchronized with a clock reference unit.
The testing to determine rectifier influence at each test station requires moving an interrupter from rectifier to rectifier and visiting each of the test stations once for each influencing rectifier. For example, if four rectifiers influence a specific length of pipeline, the interrupter will have to be moved four times and each of the test stations will have to be visited four times. If the “instant off” value also needs to be measured at each test station, it will be necessary to install interrupters into all four rectifiers in order to cycle the rectifiers on and off simultaneously. A fifth visit must then be made to each of the test stations to measure the “instant off” pipe-to-soil potential. Currently no device is available that will allow measurement of the influence from each rectifier and the “instant off” pipe-to-soil potential without having to go through each measurement sequence described above.
In addition to measuring “instant off” pipe-to-soil potential at each test station (typically spaced 1 mile apart), sometimes it is desirable to measure “instant off” pipe-to-soil potentials at regular intervals between test stations using a methodology known as a close interval survey (CIS). A CIS is performed when the data collected at test stations alone is deemed inadequate and a higher density of data points is required. A CIS is typically performed on a pipeline using a portable pipe-to-soil measurement unit connected to a test station with the reference electrode on the portable pipe-to-soil measurement unit being manually inserted into the ground at spaced intervals between adjacent test stations and a pipe-to-soil measurement taken at each interval. The spacing of data collection points on a CIS varies, but 2.5 to 5 foot intervals are typical. At present, there is also no way of obtaining the rectifier influence in conjunction with a CIS because during a CIS, the interrupters are programmed to simultaneously switch all the rectifiers either all on or all off.
The present invention overcomes the deficiencies of the prior art.